Progressing cavity hydraulic motors and pumps (also known in the art as Moineau style motors and pumps) are well known in subterranean drilling and artificial lift applications, such as for oil and/or gas exploration. Such progressing cavity motors make use of hydraulic power from drilling fluid to provide torque and rotary power, for example, to a drill bit assembly. The power section of a typical progressing cavity motor includes a helical rotor disposed within the helical cavity of a corresponding stator. When viewed in circular cross section, a typical stator shows a plurality of lobes in the helical cavity. In most conventional Moineau style power sections, the rotor lobes and the stator lobes are preferably disposed in an interference fit, with the rotor including one fewer lobes than the stator. Thus, when fluid, such as a conventional drilling fluid, is passed through the helical spaces between rotor and stator, the flow of fluid causes the rotor to rotate relative to the stator (which may be coupled, for example, to a drill string). The rotor may be coupled, for example, through a universal connection and an output shaft to a drill bit assembly. Alternatively, in pump applications, the rotor may be driven by, for example, electric power, in which case fluid may be caused to flow through the progressing cavities.
Conventional stators typically include a helical cavity component bonded to an inner surface of a steel tube. The helical cavity component in such conventional stators typically includes an elastomer (e.g., rubber) and provides a resilient surface with which to facilitate the interference fit with the rotor. Many stators are known in the art in which the helical cavity component is made substantially entirely of a single elastomer layer.
It has been observed that during operations, the elastomer portions of conventional stator lobes are subject to considerable cyclic deflection, due at least in part to the interference fit with the rotor and reactive torque from the rotor. Such cyclic deflection is well known to cause a significant temperature rise in the elastomer. The temperature rise is known to degrade and embrittle the elastomer, eventually causing cracks, cavities, and other types of failure in the lobes. Such elastomer degradation is known to reduce the expected operational life of the stator and necessitate premature replacement thereof. Moreover, the cyclic deflection is also known to reduce torque output and drilling efficiency in subterranean drilling applications. One solution to this problem has been to increase the length of power sections utilized in such subterranean drilling applications. However, increasing stator length tends to increase fabrication complexity and may also tend to increase the distance between the drill bit and downhole logging sensors. It is generally desirable to locate logging sensors as close as possible to the drill bit, since they are intended to monitor at-bit conditions, and they tend to monitor conditions that are remote from the bit when located distant from the bit.
Stators including a comparatively rigid helical cavity component have been developed to address these problems. For example, U.S. Pat. No. 5,171,138 to Forrest and U.S. Pat. No. 6,309,195 to Bottos et al. disclose stators having helical cavity components in which a thin elastomer liner is deployed on the inner surface of a rigid, metallic stator former. The '138 patent discloses a rigid, metallic stator former deployed in a stator tube. The '195 patent discloses a “thick walled” stator having inner and outer helical stator profiles. The use of such rigid stators is disclosed to preserve the shape of the stator lobes during normal operations (i.e., to prevent lobe deformation) and therefore to improve stator efficiency and torque transmission. Moreover, such metallic stators are also disclosed to provide greater heat dissipation than conventional stators including elastomer lobes.
While comparatively rigid stators have been disclosed to improve the performance of downhole power sections (e.g., to improve torque output), fabrication of such rigid stators is complex and expensive as compared to that of the above described conventional elastomer stators. Most fabrication processes utilized to produce long, internal, multi-lobed helixes are tooling intensive (such as helical broaching) and/or slow (such as electric discharge machining). As such, rigid stators of the prior art are often only used in demanding applications in which the added expense is acceptable.
Various attempts have been made to address the above-mentioned difficulties associated with rigid stator fabrication. For example, U.S. Pat. No. 6,543,132 to Krueger et al. discloses methods for forming a rigid stator about an inner mandrel having a helical outer surface. The mandrel is then removed leaving a longitudinal member having an inner profile defined by the outer profile of the mandrel. U.S. Pat. No. 5,832,604 to Johnson et al. discloses a rigid stator formed of a plurality of duplicate disks including an inner cavity having a plurality of lobes. The discs are assembled into the form of a stator by stacking on a mandrel such that the discs are progressively rotationally offset from one another. The stack is then deployed in a stator tube. U.S. Pat. No. 6,241,494 to Pafitis et al. discloses a non elastomeric stator including a plurality of stainless steel sections that are aligned and welded together to form a stator of conventional length. Nevertheless, despite these efforts, there exists a need for yet further improved stators for progressing cavity drilling motors, and in particular improved rigid stators and methods for fabricating such rigid stators.